CRISPR-Cas9 is well-known as a genome-editing tool that lets scientists rewrite DNA, but it originally evolved in bacteria. So what does CRISPR do in nature? And by what mechanism? This animation explains how the CRISPR-Cas immune system works to defend bacteria from viruses called bacteriophages.
Animated by Janet Iwasa for the Innovative Genomics Institute.
Bacteria use an adaptive immune system called CRISPR to acquire protection from viruses. When a virus injects its DNA into a bacterial cell during infection, a piece of the viral DNA is integrated into the bacterial genome—its genetic program—at a site called ‘CRISPR’ that
Includes a series of repeated sequences that flank each inserted piece of viral DNA. This creates a permanent genetic record of past infections. Then, the sequences are copied into strands of RNA, DNA’s chemical cousin, and the RNA is chopped into functional units, each containing one viral
Sequence plus the flanking repeat. The chopping process requires a separate RNA called ‘tracrRNA’ that together bind to the Cas9 protein, a component of CRISPR immunity, to form a surveillance complex. Upon finding DNA with a matching sequence, the Cas9–RNA machine holds onto the matching DNA sequence,
Unwinds it, and cuts each strand of the DNA double helix. In bacteria, the broken viral DNA is chopped up and destroyed by other proteins, thereby halting the infection.
